1. Field of the Invention
This invention relates to reflective light valve systems and more particularly concerns contrast enhancement and ghost elimination for a liquid crystal light valve projection system.
2. Description of the Related Art
The liquid crystal light valve is a thin film, multi layer structure comprising a liquid crystal layer, a dielectric mirror, a light blocking layer and a photo-responsive layer sandwiched between two transparent electrodes. In an LCLV projection system, a polarized projection beam is directed through the liquid crystal layer to the dielectric mirror. An input image of low intensity light, such as that generated by a cathode ray tube, is applied to the photo-responsive layer, thereby switching the electric field across the electrodes from the photo-responsive layer onto the liquid crystal layer to activate the liquid crystal. Linearly polarized projection light, from a high power light source, passes through the liquid crystal layer and is reflected from the dielectric mirror to be polarization-modulated in accordance with the light information incident on the photo-responsive layer. Therefore, if a complex distribution of light, for example a high resolution input image, is focused onto the photoconductor surface, the device converts the image into a replica image which can be reflected for projection with magnification to produce a high brightness image on a viewing screen. Projection systems of this type are described in several U.S. patents, including U.S. Pat. No. 4,650,286 to Koda, et al for Liquid Crystal Light Valve Color Projector, U.S. Pat. No. 4,343,535 to Bleha, Jr. for Liquid Crystal Light Valve, U.S. Pat. No. 4,127,322 to Jacobsen, et al for High Brightness Full Color Image Light Valve Projection System, and U.S. Pat. No. 4,191,456 to Hong et al for Optical Block for High Brightness Full Color Video Projection System.
The very high brightness of the high power light source employed in such systems introduces problems that may degrade contrast of the projected image and provide a display of ghost images of the light source. Although the lens elements of the projection lens are provided with anti-reflective coatings, and thus reflect but a very small percentage of light impinging thereon, even this small percentage of reflected light from the high brightness source is sufficient to be reflected by the OFF state of the light valve back onto the projection screen. This causes ghosts which are unwanted images of the arc lamp light source and its reflector at the projection screen and are most noticeable in black areas of the screen when only a portion of the screen is white. The unwanted ghost is that portion of light retroreflecting from the air glass interface of a lens element, which then forms a focused image of the arc lamp back on the light valve surface. This image is reflected by the liquid crystal light valve through the lens and continues onto the screen. The problem is related to the lens element surface of all lenses which form arc lamp images back onto the valve mirror which then are imaged by the projection lens onto the screen.
Ghost image intensity has been reduced by means of high efficiency, anti-reflection coatings on the lens elements. However, even a fraction of one percent of the very bright arc lamp source can still be highly visible when projected on to dark areas of the screen and will result in an overall loss of contrast and possible distracting image of the lamp and its reflector on the screen.
Accordingly, it is an object of the present invention to provide a reflective liquid crystal light valve projection system which avoids or minimizes above mentioned problems.